Actually, I'm not sure if it is a SmartCrawl setting after all. Is there a Multisite WP setting (or maybe Branda would help) or a code snippet to remove this feature (or maybe redirect the URL to the media URL) ?

Image permalink is not related to SmartCrawl or Multisite, that's actually default WP behavior.
In SmartCrawl you can go to Advanced Tools > URL Redirection and enable "Redirect Attachments" options but this is not exactly what you're looking for.
This will redirect images to their parent posts, so when you create a post and upload the image while in the post editor that image will redirect to that post if this option is enabled.

Do you remember how were you doing this in the past because SmartCrawl didn't had this option, it can only redirect attachments to replated posts, and that requires the image to be uploaded while editing/creating post, if you upload the image directly in Media library then it will have no redirects.

Image page uses image.php file so you can use that to customize that page.
You should put this file in your child theme and edit it from there so you can remove header, footer, sidebar and all the other stuff you don't want to show on that page.

Simply create a file named "image.php", copy & paste this code into it and upload the file to your child theme.

Note please: if you already got an "image.php" file there, make a backup of it just in case you'd need/want to restore original one.

This should display nothing but the image, although it does include some HTML elements so you could additionally style it if necessary. It also includes "alt" attribute for image. But if you want to, the code can be "stripped down" even more to display nothing but the image itself.

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